1. Field of the Invention
The present invention relates to a blow molding machine and process for blow molding heat resisting containers. More particularly, the present invention relates to a double-blow, heat set blow molding machine capable of forming "hot-fill" containers while using only a single blow molding wheel.
2. Description of the Prior Art
Plastic containers, and in particular light weight plastic bottles, are widely used to contain liquids, including carbonated and non-carbonated beverages. Because of the various manufacturing processes which must be undergone to ensure the proper "bottling" of the liquids, the containers must be able to withstand sever heat conditions without any appreciable deformation. These extreme heat conditions are encountered, for example, when the containers are filled with a hot liquid (filling temperatures of approximately 95.degree. C.) and when encountering high temperatures (60.degree.-80.degree. C.) over a long period of time (one to two hours) during pasteurization of the liquid contents. Unless the container has been properly conditioned to withstand these temperatures, the container will deform and result in an unacceptable commercial product.
One successful method of producing thermally stable containers is known in the industry as the "double-blow heat set" process. When molding plastic containers with this process, injection molded preforms are conveyed through a preheating oven to produce a desired temperature profile within the preforms. When at the proper temperature, the preforms exit the oven and are transferred to a rotating blow molding wheel. The blow molding wheel contains multiple blow molds and each mold into which a preform is loaded has a cavity which defines an intermediate configuration for the container. After initial blowing of the preform into an intermediate container configuration, the intermediate container is transferred to a conveyor that transports it through a heat treating oven.
In this oven, the applied heat causes the intermediate containers to undergo a significant degree of shrinkage and deformation. However, the deformation is not so sever that the container side walls completely collapse causing them to adhere to each other and preventing further use. Rather, the intermediate container experiences an overall amount of shrinkage which will allow the container to be re-blown.
Upon exiting the heat treating oven, the heat treated intermediate container is transferred into a mold on a second blow molding wheel. These molds include a mold cavity which defines the final configuration of the container. The heat treated intermediate container is then molded into the final configuration and the resulting containers is removed.
As seen from the above discussion, previous double-blow heat set machines and their methods of operation require that two separate blow molding wheel machines and four transfer mechanisms be used in forming the final container. Constructing, purchasing and maintaining a two wheeled, blow molding machine is itself an expensive undertaking and this translates into a costly final product.
With each wheel blow molding machines costing in the several million dollar range and with the profit margins being pennies on a container, and further in view of other limitations, shortcomings and disadvantages not specifically mentioned above, it should be apparent that there still exists a need for a more cost effective way to produce these heat resistant containers.
It is therefore a primary object of this invention to fulfill that need by providing a blow molding machine for blow molding heat resistant containers generally according to the double-blow heat set process.
Another object of the present invention is to provide a double-blow molding machine which requires only a single blow molding wheel.
A further object of this invention is to reduce the number of transfer mechanisms needed to handle the preforms intermediate and final containers in order to run the machine.
Yet another object of this invention is to provide a double blow molding machine having the variability to increase or decrease residence times in the various molds and the heat treating oven.